In recent years, with the raising awareness of environmental protection, almost every country in the world has invested in the development of alternative energy sources, and among those efforts, one alternative energy source that had been commonly agreed to be the most promising is solid oxide fuel cells.
The solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel, and in this oxidation reaction, only water as a byproduct is being given off. Thus, this class of fuel cells is characterized in that: high efficiency and low emissions.
SOFCs can be divided into two types of SOFCs depending on their geometry designs, which are the planar type and the tubular type. Generally, The power performance of the planar SOFCs is currently better than the power performance of the tubular SOFCs. However, all the SOFCs in various designs should share the advantages of high efficiency, long-term stability, and relatively low cost.
There are numerous reports and patents for various SOFC constitutions had been provided, whereas a common SOFC is composed of electrolytes, anodes and cathodes. Generally, the electrolyte is made of yttria-stabilized zirconia (YSZ), the anode is made of a cermet (Ni/YSZ) composed of nickel (Ni) and yttria-stabilized zirconia (YSZ), and the cathode is made of conductive lanthanum strontium-doped manganite (LSM, LaMnO3) with a perovskite structure.
However, since yttria-stabilized zirconia (YSZ) exhibits sufficient ion conductivity only under high temperatures larger than 900° C., and as a consequence, the sealing material and the connection material used in the solid oxide fuel cell (SOFC) must be made of high-cost materials with good high-temperature resistance. Therefore, SOFCs can be too expensive to be used widely.
Nevertheless, there are SOFCs which is designed to operate at a temperature lower than 900° C. by adopting a thinner YSZ electrolyte, but such SOFCs still suffer great energy loss due to resistance. On the other hand, an electrolyte (made of, for example, lanthanum strontium gallate magnesite (LSGM) with high ion conductivity, can be used to manufacture a solid oxide fuel cell that works at intermediate temperatures ranged from 600° C. to 800° C.
Moreover, as the temperature decreases, electrochemical activities at the cathode and anode decrease, causing polarization resistances at the cathode and anode to increase and thus inducing great energy loss.
For the planar SOFCs, they are usually being fabricated with robust supports for supporting the construction of the whole SOFC stack. The most common supports are made of a composite of ceramic and metal, and thus can be referred as cermet (ceramic-metal) supports. However, such cermet supports are disadvantageous in their high cost, difficulty to process, vulnerability to cracking and breakage, low resistance to thermal shocks, and low thermal conductivity.
Therefore, there are more and more metal-supported planar SOFCs to be developed for replacing the traditional cermet-supported planar SOFCs, and the metal-supported planar SOFCs have porous metal substrates to support themselves. There are already many methods being developed for fabricating such metal-supported planar SOFCs, such as strip casting, pulsed laser deposition (PLD), and air plasma spray (APS). Among which, APS is most common method, but it can cause the fuel cell to deform. The reason is: the porous metal substrate and the different layers of a SOFC are generally made of different materials with different thermal expansion coefficients so that they can expand differently while the layers of a SOFC are sequentially coated on the porous metal substrate by the high temperature plasma spraying in the APS process, and thus thermally induced stresses exerted between different layers of a SOFC cause a SOFC to deform as the consequence.